System and method for cooling photovoltaic cells

ABSTRACT

A cooling system for PV cells includes an evaporator configured to thermally contact the PV cells and transfer heat generated thereby to coolant in the evaporator, a condenser for receiving vaporized coolant from the evaporator and condensing the coolant to a liquid state, tubing connecting the evaporator, and the condenser in a circuit, a compressor arranged in the circuit for pumping coolant therethrough in a coolant flow direction, an active charge control apparatus arranged in the circuit between, in the coolant flow direction, the evaporator and the condenser, and a liquid flow control apparatus arranged in the circuit between, in the coolant flow direction, the condenser and the evaporator. The active charge control apparatus and the liquid flow apparatus cooperate to maintain the evaporator completely wetted by coolant and prevent coolant in the liquid state from leaving the evaporator.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/521,071, filed on Aug. 8, 2011, the contents of which are herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the cooling of photovoltaic (PV) cells, and more particularly, to systems and methods for circulating a coolant for this purpose.

BACKGROUND OF THE INVENTION

Various systems and methods are known for cooling PV cells during operation. In some instances—for example in systems using water or glycol or other liquids as a coolant—the coolant will warm up as it traverses the panels of PV cells, therefore providing greater cooling near the entrance into the panel assembly, and less cooling near the exit of the panel assembly.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to provide improved systems and methods for cooling PV cells. According to an embodiment of the present invention, a cooling system for PV cells includes an evaporator configured to thermally contact the PV cells and transfer heat generated thereby to coolant in the evaporator, a condenser for receiving vaporized coolant from the evaporator and condensing the coolant to a liquid state, tubing connecting the evaporator, and the condenser in a circuit, a compressor arranged in the circuit for pumping coolant therethrough in a coolant flow direction, an active charge control apparatus arranged in the circuit between, in the coolant flow direction, the evaporator and the condenser, and a liquid flow control apparatus arranged in the circuit between, in the coolant flow direction, the condenser and the evaporator. The active charge control apparatus and the liquid flow apparatus cooperate to maintain the evaporator completely wetted by coolant and prevent coolant in the liquid state from leaving the evaporator.

According to a method aspect, a method for cooling PV cells to increase their efficiency, and for capturing the waste heat from the PV cells, includes placing an evaporator in thermal contact with the PV cells and circulating coolant through the evaporator to remove waste heat therefrom. Circulating coolant through the evaporator includes maintaining the evaporator in a wetted state with substantially no coolant superheating.

These and other objects, aspects and advantages of the present invention will be better appreciated in view of the drawings and following detailed description of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a photovoltaic (PV) cell cooling system, according to an embodiment of the present invention, including an active charge control (ACC) apparatus, a liquid flow control (LFC) apparatus, and an evaporator;

FIG. 2 is a sectional side view of the ACC apparatus of FIG. 1;

FIG. 3 is a sectional side view of the LFC apparatus of FIG. 1;

FIG. 4 is a sectional side view of an alternate to the evaporator of FIG. 1, according to another embodiment of the present invention;

FIG. 5 is a partially sectioned bottom view of another alternate to the evaporator of FIG. 1, according to a further embodiment of the present invention;

FIG. 6 is a sectional view taken along line C-C of FIG. 5;

FIG. 7 is a partially sectioned bottom view of an additional alternate to the evaporator of FIG. 1, according to an additional embodiment of the present invention;

FIG. 8 is a sectional view taken along line A-A of FIG. 5;

FIG. 9 is a partially sectioned bottom view of a further alternate to the evaporator of FIG. 1, according to a further embodiment of the present invention;

FIG. 10 is a sectional view taken along line B-B of FIG. 5;

FIG. 11 is a partially sectioned side view of an alternate to the LFC apparatus of FIG. 1, including a cylinder 103; and

FIG. 12 is an end view of the cylinder 103 of FIG. 11, showing an inlet thereinto.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, according to an embodiment of the present invention, a PV cell cooling system 10 includes a compressor 11 that pumps refrigerant vapor from outlet 12 to a condenser 13 (flow direction of coolant represented by unlabeled arrows), where the hot refrigerant is cooled by the condenser 13, thereby delivering heat energy into the fluid 14 in tank 28. The refrigerant, being cooled by the fluid in tank 28, condenses to a liquid state within Condenser 13, and exits the condenser at outlet 15. Conduits connect the system 10 components into a complete circuit. The refrigerant is delivered to a liquid flow control (LFC) apparatus 17 via conduit 16. The refrigerant leaving the LFC apparatus is delivered to the inlet 19 of the evaporator 20 by way of conduit 18. The liquid refrigerant then contacts an evaporator wall 21 as it moves upward through an evaporator space 22. As the refrigerant evaporates in space 22, it absorbs heat from the wall 21 that is in physical and thermal contact with PV support member 27, and thereby cools the PV cells 26, which are bonded to, and in thermal contact with, support member 27. The refrigerant moving up through the evaporator space 22 is forced by an active charge control (ACC) apparatus 24, working in conjunction with LFC apparatus 17, to finish evaporating and be in vapor form only as it leaves the evaporator space 22 and proceeds via conduit 23 to the ACC apparatus 24. This is accomplished by the LFC 17 holding a fixed amount of sub-cooling in the condenser 13 while the ACC apparatus allows only vapor to leave the ACC apparatus, and more preferably only vapor that is not superheated.

Referring to FIG. 2, the ACC apparatus 24 receives refrigerant vapor at entrance 81. The refrigerant proceeds past venturi 85 which entrains liquid refrigerant into evaporator tube 82. When the vapor/liquid exits tube 82 and impinges on deflector disc 86, the liquid falls back into the liquid pool at liquid level 84, while the lighter vapor flows past the deflector disc to leave the ACC apparatus 24 at outlet 87. Thus the ACC apparatus 24 effectively separates the vaporized refrigerant from the liquid refrigerant, and only saturated vapor (not superheated) leaves the ACC apparatus 24. The temperature of the contents of the ACC tank 88 is determined by the amount of suction pressure at the compressor inlet. Therefore if superheating of the refrigerant starts to occur in the evaporator 20, the superheated vapor going through the evaporator tube 82 will be warmer than the liquid reserve in the ACC apparatus 24, and will evaporate refrigerant from the reserve of liquid in ACC apparatus 24, and place more refrigerant into active circulation through the whole refrigerant circuit. This additional refrigerant will further “flood” or “wet” the evaporator 20 until all superheating is eliminated. Conversely, if too much refrigerant is in circulation, some amount of liquid will pass through and out of evaporator space 22, but when any liquid reaches the ACC apparatus 24, it simply falls back into the reserve pool to eliminate the excess of refrigerant in active circulation. Thus the ACC apparatus ensures that the space 22 in the evaporator 20 is constantly “flooded” or “wetted,” and therefore superheating does not occur in the evaporator 20.

The ACC apparatus 24 and LFC apparatus 17 operate together to ensure the entire evaporator space 22 has liquid refrigerant present throughout the evaporator 20, from entrance 19 to the outlet 23, and therefore heat is uniformly absorbed from wall 21 of the evaporator 20, with the result that the cooling of the PV support member 27, and in turn the cooling of the PV cells 26, is uniform throughout the panel of PV cells. The refrigerant passes from the ACC apparatus 24 and on to the compressor 11 to repeat the cycle again.

Referring to FIG. 3, extensive testing has shown that a refrigerant circuit is more efficient if subcooling in the condenser is held to a low value, preferably in the range of 1 to 8 degrees Fahrenheit, with the optimum amount of subcooling being about 4 degrees Fahrenheit. The LFC apparatus 17 (a subcool control valve (SCV), in the FIG. 3 embodiment) is designed to achieve that purpose when used in concert with ACC apparatus 24. In the SCV 17, liquid refrigerant from the condenser 13 flows in at inlet tube 61, and on and upward between outer tube 62 and outlet tube 63, and onward to main orifice 65 and side orifice 66 in orifice plug 68. The liquid is metered at the orifices 65 and 66, and after metering and expanding, leaves the valve via outlet tube 63. Dome 67 and diaphragm 66 form a sealed cavity 72 which contains a controlling liquid refrigerant 70. The controlled fluid, represented by the flow arrows, makes physical and thermal contact with diaphragm 66, which in turn requires the fluid 70 to assume approximately the same temperature and pressure as controlled fluid flowing to the orifices.

If the incoming controlled fluid is overly subcooled, the controlling fluid 70 will also become more subcooled which reduces the pressure on top of the diaphragm 66, causing the diaphragm to flex upward, thereby opening the main orifice, and increasing the flow of the controlled fluid from the condenser 13 and reducing the amount of subcooling. Conversely, if the subcooling becomes too little, the pressure above the diaphragm will decrease and increase the amount of subcooling, all with the result that a predetermined amount of subcooling is maintained in the condenser. The thickness and flexibility of the diaphragm 66 are the primary factors that determine the amount of subcooling that results. The side orifice 73 provides a minimum flow and prevents instability and a possible unintentional shutdown of the system. Note that the diaphragm stop 71 serves to prevent overstressing the diaphragm under various unusual conditions. The LFC apparatus of FIG. 3 has an additional feature and advantage, in that a “built in” heat exchanger is formed by outer tube 62 and outlet tube 63, wherein the metered, expanded, and chilled refrigerant leaving through outlet tube 63 makes thermal contact with the incoming and un-metered liquid inside outer tube 62 via the wall of outlet tube 63, to thereby provide inverse thermal feedback to the incoming liquid, for stabilizing the LFC apparatus.

Advantageously, the present method uses a refrigerant in connection with PV cell cooling. As used herein, a refrigerant is a specific type of coolant which evaporates as it traverses through an evaporator. Thus, while a “coolant” could be any fluid capable transferring heat away from a heat source, a “refrigerant” is more specifically a coolant that will undergo a phase change while doing so. In other words, as the terms are used herein, all refrigerants are coolants, but all coolants are not necessarily refrigerants. By keeping the evaporator wetted with refrigerant at its vaporization point, an even temperature profile will be experienced by the PV cells being cooled. With this in mind, water and glycol are not preferred coolants for many PV cell applications as they will remain subcooled liquids under expected operational conditions.

As will be appreciated from the foregoing, the refrigerant circuit which drives the refrigerant through the evaporator is designed to continuously supply the required amount of refrigerant to the evaporator that will result in the “wetting” or “flooding” of the entire evaporator. When an evaporator is flooded, there is minimal difference in temperature of the evaporator from the entrance end to the exit end of the evaporator. Flooding the evaporator eliminates any superheating of the refrigerant within the evaporator. Eliminating superheating results in a more uniform cooling of the PV panel. Overheating of even one PV module in multiple modules connected in series can result in limiting the output power of the series of modules, and could result in damage to the overheated module. The uniform cooling provided by the present invention prevents such results. Eliminating superheating also results in a higher “suction pressure” and a cooler, more dense refrigerant vapor at the compressor entrance. These two factors result in more refrigerant pumped per stroke of the compressor, and improvement of efficiency of the refrigerant circuit.

The heat removed from the PV panel(s) may typically be delivered to a condenser for useful heating of water, or to the condenser of an air handler for the heating of air. The condenser may be used in other heating applications. This arrangement allows for the more effective recapture of what would otherwise be waste heat generated by the PV panels. Thus, the PV cell cooling system is not only able to enhance efficiency of the PV cells—thereby reducing electricity that needs to be supplied by other (potentially less “green”) sources, it can also simultaneously recapture more waste heat for other applications that would otherwise require another (again, potentially less “green”) heat source.

The above described embodiment is presented for exemplary and illustrative purposes; the present invention is not necessarily limited thereto. For example, referring to FIG. 4, the structure therein replaces the structure of 19, 20, 21, 22, 26, and 27 in FIG. 1, with like reference numerals (followed by an “A”) referring to analogous elements. Support member 27A serves multiple purposes. For example, it forms one wall 21A of the evaporator chamber 22A, and secondly it serves as the support member 27A for the PV cells 26A, thereby eliminating the need for a separate wall member 21, and reducing the complexity, weight, and cost of the assembly, while improving the thermal conductivity between the evaporating refrigerant and the PV cells 26A. Thus, as the refrigerant circuit absorbs unwanted heat from the PV cells, it delivers it as useful heat to the condenser in tank 28, or to any condenser for delivering useful heat.

In other examples, FIGS. 5-10 show various alternative structures that may be used for PV cooling, as alternatives to the simplified evaporators 20, 20A as shown in the refrigerant circuit of FIGS. 1 and 1A.

Referring to FIGS. 5 and 6, a cooled panel A includes serpentine tubing 32 attached to tubing support member 30 by thermally conductive bonding material 34. The tubing support member 30 is in physical and thermal contact with PV cells 31. The refrigerant evaporates as it flows through the tubing 32 from inlet 36 through passage 35 to outlet 37, and tubing support member 30 is cooled, which in turn cools PV cells 31. Tubing support member 30 is made such that it supports the PV cells, and the support member 27 in FIG. 1 is eliminated.

Referring to FIGS. 7 and 8, a cooled panel B includes a corrugated member 42 joined to panel support member 40 by seam welds 43 thereby forming fluid passages 44 such that a refrigerant flowing through the passages from entrance 45 to exit 46 extracts heat from member 40 and in turn from PV cells 41. Support member 40 also serves as one wall of the cooled panel B.

Referring to FIGS. 9 and 10, a cooled panel C uses staggered seam-welds 53 to attach corrugated member 52 to panel support member 50 thereby forming a serpentine pathway 54 where the coolant enters at inlet 55 and after traversing the pathway exits the panel at outlet 56, thereby cooling PV cells 51. Support member 50 also serves as one wall of the cooled panel C.

In a further alternative, referring to FIGS. 11 and 12, a different LFC apparatus—LFC apparatus 17A—is used. The LFC apparatus 17A is used to hold a fixed amount of subcooling in the condenser 13, thereby cooperating with the ACC 24, but that amount of subcooling is normally zero, because a small trickle of uncondensed vapor is required to arrive at the LFC 17A to operate the float 107. The body of the LFC 17A in FIG. 7 is a cylinder 103 which is enclosed with end caps 111, thus providing a closed chamber for float 107. Liquid refrigerant enters the chamber at entrance tube 113, and inlet 112. Cylinder 103 fills with liquid to a height determined by the amount of vapor refrigerant arriving with the liquid. The float 107 is attached to metering segment 116 by float attachment 110 and attaching rod 109, thereby making the metering segment 116 swivel on swivel pin 102. The metering segment 116 being otherwise generally circular, is flat at segment 116. When little or no vapor arrives at valve 117, the cylinder 103 fills with liquid, indicating that liquid is backing up in the condenser 113. As cylinder 103 fills with liquid, float 107 rises.

Raising the float 107 presents the flat segment 116 to the valve orifice which is centered in valve plug 105, which in effect opens the LFC apparatus 17A to release liquid refrigerant from condenser 13. When all liquid is released from condenser 13, vapor arrives in cylinder 103 and the float 107 is forced downward by the vapor rising to the top of cylinder 103, which closes the valve to require more complete condensing of the refrigerant within the condenser. Equilibrium is reached when just a small trickle of bubbles (vapor) arrives from the condenser, and just enough of the flat on the metering segment 116 is presented to the outlet orifice of LFC apparatus 17A to maintain zero subcooling in condenser 13. After the refrigerant is metered and expanded in metering plug 105, it proceeds through outlet tube 101 and 106 which is formed to make thermal contact with cylinder 103 using a silver braze 114, thus LFC apparatus 17A. The metered and expanded refrigerant exits the LFC apparatus 17 at outlet 115.

The foregoing is not intended to be an exhaustive list of alternatives. Rather, those skilled in the art will appreciate that these and other modification, as well as adaptations to particular circumstances, will fall within the scope of the invention as herein shown and described and of the claims appended hereto. 

1. A cooling system for photovoltaic (PV) cells comprising: an evaporator configured to thermally contact the PV cells and transfer heat generated thereby to coolant in the evaporator; a condenser for receiving hot coolant from the evaporator and extracting heat therefrom, coolant leaving the condenser in the liquid state; conduits connecting the evaporator, and the condenser in a circuit; a compressor arranged in the circuit for pumping coolant therethrough in a coolant flow direction; an active charge control apparatus arranged in the circuit between, in the coolant flow direction, the evaporator and the condenser; and a liquid flow control apparatus arranged in the circuit between, in the coolant flow direction, the condenser and the evaporator; wherein the active charge control apparatus and the liquid flow apparatus cooperate to maintain the evaporator completely wetted by coolant and prevent coolant in the liquid state from leaving the evaporator.
 2. The system of claim 1 wherein one wall of the evaporator is also a support member for the PV cells, placing the evaporator in direct thermal contact with the PV cells.
 3. The system of claim 1 wherein the liquid flow control apparatus includes: a cylinder; and a float arranged in the cylinder; wherein the cylinder fills with liquid to a height determined by the amount of vaporized coolant arriving from the condenser, thereby altering a height of the float, and the height of the float determining the rate of liquid flow through the liquid flow control apparatus, and working in concert with the active charge control apparatus holds the amount of subcooling in the condenser to approximately zero, which ensures that all non-circulating liquid coolant in the system is stored in the active charge control apparatus, with the result that zero superheat is maintained in the evaporator.
 4. The system of claim 1, wherein the liquid flow control apparatus includes: an inlet tube; an outlet tube; an outer tube surrounding the outlet tube; a sealed cavity containing a controlling fluid, the cavity being formed by a dome and an impervious flexible diaphragm, said diaphragm being subjected to the pressure and temperature of the incoming controlled fluid from the condenser against its underside, and said diaphragm being subjected to the temperature and pressure of the controlling fluid on its top side, such that the diaphragm responds to the temperature and pressure of the controlled fluid to flex the diaphragm upward to open the valve when there is excess subcooling of the controlled fluid, with result that subcooling of the liquid from the condenser is held within a predetermined range and the LFC apparatus cooperates with the ACC apparatus to prevent superheat in the evaporator.
 5. The system of claim 4, wherein the predetermined range is zero to eight degrees Fahrenheit, thus cooperating with the active charge control to prevent superheat in the evaporator, and therefore uniform cooling of the PV cells.
 6. The system of claim 1, wherein the evaporator includes serpentine tubing thermally bonded to a structural member which is in thermal contact with the PV cells.
 7. The system of claim 1, wherein the evaporator includes corrugated metallic panels joined to one of a flat metal panel or another corrugated metallic panel to form refrigerant passages in thermal contact with the PV cells.
 8. A method for cooling photovoltaic (PV) cells to increase their efficiency, and for capturing the waste heat from the PV cells, the method comprising: placing an evaporator in thermal contact with the PV cells; and circulating coolant through the evaporator to remove waste heat therefrom; wherein circulating coolant through the evaporator includes maintaining the evaporator in a wetted state with substantially no coolant superheating.
 9. The method of claim 8, wherein maintaining the evaporator in a wetted state with no coolant superheating includes minimizing subcooling in coolant supplied to the evaporator with a liquid flow control apparatus arranged between a condenser an evaporator inlet.
 10. The method of claim 9, wherein maintaining the evaporator in a wetted state with no coolant superheating further includes preventing superheating with an active charge control apparatus arrange between an evaporator outlet and a compressor inlet.
 11. A PV cell cooling system comprising a refrigerant circuit, including a compressor, a condenser, a liquid flow control, an evaporator, and an active charge control, wherein the evaporator is in thermal contact with Photovoltaic cells for cooling the PV cells to achieve at least one of, eliminating the loss of efficiency that results from overheating of the PV cells, or reducing the temperature of the PV cells to a temperature below the ambient air temperature, to further increase efficiency and to increase the electrical output of the PV cells.
 12. The PV cooling system of claim 11, wherein the evaporator is comprised of serpentine tubing, and which tubing is thermally bonded to a structural member which is in thermal contact with the PV cells that are thereby cooled.
 13. The PV cooling system of claim 11, wherein one wall of the evaporator also serves as the support member for the PV cells, thereby placing the evaporator in direct thermal contact with the PV cells.
 14. The PV cooling system of claim 13, wherein the evaporator is comprised of serpentine tubing, and which tubing is thermally bonded to a structural member which is in thermal contact with the PV cells that are thereby cooled.
 15. The PV cooling system of claim 13 wherein the evaporator is comprised of corrugated metallic panels joined to one of a flat metal panel or another corrugated metallic panel to form refrigerant passages wherein the refrigerant evaporates to extract heat from the PV cells.
 16. The PV cooling system of claim 11, wherein the liquid flow control is comprised of an inlet tube, an outlet tube, an outer tube surrounding the outlet tube, a sealed cavity containing a controlling fluid, the cavity being formed by a dome and an impervious flexible diaphragm, said diaphragm being subjected to the pressure and temperature of the incoming controlled fluid from the condenser against its underside, and said diaphragm being subjected to the temperature and pressure of the controlling fluid on its top side, such that the diaphragm responds to the temperature and pressure of the controlled fluid to flex the diaphragm upward to open the valve when there is excess subcooling of the controlled fluid, with result that the subcooling of the liquid from the condenser is held at a low and predetermined value, within a range of zero to 8 degrees Fahrenheit.
 17. The PV cooling system of claim 16, wherein the evaporator is comprised of serpentine tubing, and which tubing is thermally bonded to a structural member which is in thermal contact with the PV cells that are thereby cooled.
 18. The PV cooling system of claim 16, wherein the evaporator is comprised of corrugated metallic panels joined to one of a flat metal panel or another corrugated metallic panel to form refrigerant passages wherein the refrigerant evaporates to extract heat from the PV cells.
 19. A method for cooling PV cells to increase their efficiency, and for capturing the waste heat from the PV cells for useful purposes, wherein a refrigerant circuit transfers excess heat from the PV cells to a fluid such as water, glycol, air or other fluid where the useful heat is needed, and wherein a compressor pumps a refrigerant vapor into and through a condenser, wherein the refrigerant vapor condenses to a liquid as it passes through the condenser, and the liquid refrigerant leaving the condenser is then forced through a liquid flow control and on to the inlet of an evaporator, and the liquid refrigerant evaporates back to vapor as it passes through the evaporator and wherein the evaporator is in thermal contact with the PV cells, and wherein the waste heat from the PV cells is absorbed into the refrigerant as it evaporates back to a vapor, and wherein the refrigerant vapor is forced onward to the inlet of an active charge control, and wherein the active charge control works in concert with the liquid flow control to trap any liquid refrigerant that may reach the active charge when too much refrigerant is in active circulation in the system, and wherein the active charge control serves to evaporate and place more refrigerant into active circulation when too little refrigerant is in active circulation in the system and the evaporator is not fully wetted and allowing superheating to occur in the evaporator, whereby the result is that the evaporator is fully wetted for uniform cooling of the PV cells, and only vaporized refrigerant, with no superheat is forced onward from the outlet of the active charge control to the inlet of the compressor, thereby completing the cycle, while extracting waste heat from the PV cells and delivering the useful heat at the condenser.
 20. The method of claim 19 wherein one wall of the evaporator also serves as the support member for the PV cells, thereby placing the evaporator in direct thermal contact with the PV cells.
 21. The method of claim 19, wherein the liquid flow control is comprised of an inlet tube, an outlet tube, an outer tube surrounding the outlet tube, a sealed cavity containing a controlling fluid, the cavity being formed by a dome and an impervious flexible diaphragm, said diaphragm being subjected to the pressure and temperature of the incoming controlled fluid from the condenser against its underside, and said diaphragm being subjected to the temperature and pressure of the controlling fluid on its top side, such that the diaphragm responds to the temperature and pressure of the controlled fluid to flex the diaphragm upward to open the valve when there is excess subcooling of the controlled fluid, and to close the valve when there is too little subcooling of the controlled fluid, with a result that the subcooling of the liquid from the condenser is held at a low and predetermined value. 